I hope you get it resolved. I know how frustrating things like this can be. I'm presently doing battle Royal with
another one of those crazy Walbro LME carbs on a Tec HH120. I've done everything I can think of, and it's still
doing the same thing. Aaargh

The magnet on the flywheel needs to be decent, and clean, and properly gapped . Also you need proper crank speed, that mag doesn't want to spark unless the engine turns 400 rpm or more , this wad killing me on mine too, so don't feel you ate the only one.
If it cranks slow you will never get it to start.....
Is this engine horizontal, or verticle ?

Patience. Personally I enjoy the challenge of hands-on troubleshooting of any engine that will not start. Second hand troubleshooting not so much.

While fuel, air and spark are all required to make a gasoline engine run it is very important that all three are delivered in the proper amount and at the proper time.

The fuel and air must be at the proper mixture level. Too much or two little fuel in the can cause a no start. Also the fuel and air mixture must be properly compressed or it will not "explode". Perhaps the rings or valves are badly worn since the engine will run better wide open than at an idle.

The spark must be hot enough and properly timed to ignite the compressed air/fuel mixture. A weak spark may be caused by coil/magneto, improper air gap, partially or completely sheared flywheel key (often caused by an engine backfiring one time too many), weak magnets on the flywheel (they do go bad), dirty or improperly adjusted points or a bad condenser all can and do cause problems. A spark plug that has been "flooded" in cold weather will very often never work again and will require replacement ( ask any snowmobile owner).

One more thing on spark. Always test with the plug in and use a spark tester. A weak spark may jump the gap laying on the engine but will not jump the gap under compression.
It takes higher voltage to fire the plug under compression.

I just fought the same problem on my Massey. Brand new motor, gas, fire, air still no run. My spark was not blue and that problem was grounding. I ran a ground wire from the - battery terminal to the motor and that fixed that problem. Other problem was I had the vari speed in gear, (hangs his head in shame). Is there any drag on the output shaft of the motor? Is the starter motor turning over fast enough? Maybe a drop of oil on the bearings will help that.

I have not been out to the shop yet, but everyone has great suggestions. I rewired the tractor myself and went from stock 6 gauge battery cables to 4 gauge (bigger wire). That upgrade alone caused the engine to turn over faster with the starter, so starting RPM is no issue. Magnets on the flywheel.. when I replaced the magneto I hit all of them with a little wire brush to knock the rust off, and gapped the magneto to 11 or 12 thousandths (instruction sheet said 10-14 thousandths). The thing that interests me the most is this ground that was run from engine to battery for better spark. I might look at that today, instead of spark plug gap. I got a lot to work on to see what I can do, so I better get to it.

It does run better at higher RPMs when it has sat for a while (gas clot lol). I usually run it til it clears up.

Alright, verdict is in... I got it to run and guess what... I think it's the Carb. Finally got to see a spark using a J8 spark plug, wasn't blue, but it was a a good spark on a plug cooler than the J19 so should be satisfactory. The first thing out of the shoot its throwing gas out of the carb, which I believed meant it was flooded... so I pulled the plugs... not even wet... hmmm... went and got the gas can and threw a shot of gas down the throat of the carb. Throttled up some, turned the key, pulled the choke out and VROOOM.. and it continued to run, and now idle down, runs rough lower idle, but I could not get it adjusted right, and thus gave up because I figure the carb needs to be rebuilt anyway. I did stall it once, but it restarted, so I figure it restarts because it had gas in the chambers residual or something, I dunno, need to order a carb kit though.

Pull the carb and give a GOOD cleaning. This includes tearing it all down and at the very least...achase all the passages with a fine wire, spray carb cleaner, and compressed air. Also...order a new float needle. There is likely a layer of varnish that is keeping your current float either: 1) hung up, allowing fuel to continue to flow which explains the need for a WOT position when starting OR 2) the Viton tip (if equipped) is hard and no longer sealing in the float seat. If there is no Viton tip on the float needle, make sure that the tip is as sharp as a sewing needle. It is VERY common for the tip to be broken off and in that event, not able to seal the float needle seat, correctly.

I'm a "self professed" Carb Geek. Lost count on the number of complete rebuilds I've done over the past few decades but I remember it was well passed the 200 mark. :blush2:

I sure hope I have it all tracked down, because after the Caterpillar is starting and running well, and I finish the front bumper, winch wiring, and lighting.... big list lol. I am going to start building a sleeve hitch wheel lift similar to Kenny's, only shorter because I don't want to have to set 80 pound chunk of railroad track on my front bumper to be able to use it. Oh, I still have to mount my new tires, holy cow, this month is going fast!! lol

Casey , I don't know what you have as far as carb cleaners but I've have pretty good luck with Gunk Carb cleaners , I have both the can to dip the parts in and the sprays The sprays seem to work better then some of the other brands too , AlGunk Details

Yesterday was spent working on the Caterpillars carb. I had to unbolt it from the manifold with a Taiwan 7/16" wrench, and you ask why... the bolts were tight, and I had to grind the box end until it looked like it could break just to get it on the bolt!! But anyway, I got the carb off and disassembled, then I dunked it in the carb cleaner for a couple hours. Then I wire brushed it, and reassembled the carb with the kit Bob sold me, thanks again Bob. I did not install the new needle seat, decided it wasn't worth the hassle since the one int he carb looked fine. Yesterday I left off by reinstalling the carb on the tractor with my custom wrench lol.

Fast forward to today, 10 minutes ago. Attempt one to restart was cut short, and you might ask why... I am still learning and glad I left the air cleaner setup off.. because I had a leak from the middle of the carb. Solution, snug the bolts up a little more and presto, no leak during attempt two, which resulting in a vroom that continued. I had just put the needle in some random number of turns so I spent a few minutes adjusting it until the tractor seemed to be running decent, and didnt stall out between high idle and a decent spend downthrottle. So I have shut the Caterpillar down with a victory, and plan to go back out to start it again after bit. There is a slight clack but I don't think it is a problem, never heard it before because of higher RPMs. Before I start it up again I will doublecheck the oil. it is so exciting when it runs, man I am excited now lol.